I agree with Mark. Stainless steel
needs oxygen to oxidize the chromium that is part of the alloy. This is what passivation is in reality This oxidized surface layer is what makes stainless stain less. Without this chromium oxide layer it's just like any other steel. It will rust when exposed to chlorides such as salt water
. If something is scratching the chromium oxide surface and there is no oxygen to oxidize the chromium then the iron will rust.
One possible solution would be what Mark suggested. Maybe you can raise the teak by placing plastic washers between the bowsprit and the teak planks where they are bolted. That would allow air to circulate around the stainless. Also, a freshwater rinse would have an easier time removing salts. McMaster Carr sells all kinds of plastic washers. Try to find some that are high density polyethylene or Delrin.